1. Field of the Invention
The present invention relates to a polygon mirror molded by injection molding and a molding die used to mold the same.
2. Description of the Related Art
A polygon mirror, which switches or scans reflected light by rotating, is provided with a plurality of reflective surfaces. The polygon mirror is molded by injection molding using a die. A polygon mirror manufacturing method is described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 8-238682 and Jpn. Pat. No. 2619244.
FIG. 10 illustrates the manufacturing method described in Jpn. Pat. Appln. KOKAI Publication No. 8-238682.
A polygon mirror 110 includes a flat portion 112 and a plurality of reflective surfaces 114 that are coupled to the flat portion 112 so as to be arranged individually on its outer peripheral side faces. The flat portion 112 is coupled to an intermediate portion of the reverse side of each reflective surface 114. An annular boss portion 112a in which a rotating body is mounted is formed in the central part of the flat portion 112.
A die for injecting the polygon mirror 110 of this type includes a sprue 141 and a disc gate 140 that extends horizontally from a terminal end of the sprue 141. The sprue 141 extends along a rotation axis CL of the polygon mirror 110 to be molded. Numeral 145 denotes a sliding die for molding the reflective surfaces 114 of the polygon mirror 110.
In molding the polygon mirror 110, a molten resin material is injected to the sprue 141 under a given pressure. After the resin material is filled into a cavity of the die through the disc gate 140, the die is cooled to solidify the resin material. Thereafter, the die is opened, and a molded piece is ejected by an ejector pin 147, whereupon the injection molding terminates.
After the molding, the disc gate 140 is removed, whereupon an annular boss portion 112a is formed. The inner surface of the boss portion 112a serves as a mounting surface for the rotating body. In filling the resin material with this structure, the resin material is fed radially outward at a time from the rotation axis CL in the central part, so that weld lines cannot be easily produced.
FIGS. 11A and 11B illustrate the manufacturing method described in Jpn. Pat. No. 2619244.
A polygon mirror 210 includes a boss portion 212a, plate-like portions 212, and reflective surfaces 214. The boss portion 212a is formed in the central part and serves as a mounting portion for a rotating body. A plurality of (e.g., eight) plate-like portions 212 extend radially from the boss portion 212a. The reflective surfaces 214 are formed individually on the respective distal ends of the plate-like portions 212 that extend from the boss portion 212a. A die is provided with a cavity that forms the boss portion 212a, plate-like portions 212, and reflective surfaces 214.
In order to fill a molten resin material into the cavity of the die, a sprue 241 extends corresponding to the central part of the boss portion 212a, as shown in FIG. 11B. Runners 242 extend radially from the sprue 241. The runners 242 extend corresponding to the plate-like portions 212, individually. The respective distal end portions of the runners 242 serve individually as pin gates 245 through which the resin material is fed into the cavity. The pin gates 245 as many as the reflective surfaces 214 are provided corresponding to the reflective surfaces 214, individually. The pin gates 245 are arranged substantially in the central part with respect to the transverse direction of the plate-like portions 212. Thus, the pin gates 245 are located individually on bisectors perpendicular to their corresponding reflective surfaces 214.
With this construction, the amount of the resin material injected through each pin gate 245 and the balance of pressure are fixed. The respective entire areas of the reflective surfaces 214 are located at substantially equal distances from the pin gates 245. Therefore, the molten resin material flows uniformly and spreads equally. Thus, weld lines formed between the adjacent reflective surfaces 214 are produced along ridges 235 (FIG. 11A) or boundaries between the plate-like portions 212, and weld lines on the reflective surfaces 214 can be prevented from being produced.